Salvaged some great beams from an old barn being torn down. The nails are solid rust and break off under the surface of the wood when pulled. Anyone know of a tool or technique to find them and remove before they are machined?

A metal detector will find them. There are MD wands that are sold to woodworkers specifically for this purpose.

Getting them out is a different story. You might be able to use a plug cutter to burrow around the nail and maybe get down to good metal that you can effectively pull. At the very least, you can get to better metal and drive it deeper, beyond the depth of machining processes as long as you're only surfacing the material. If you're going to resaw it, then your best tool for the job will be a band saw that will cut through the nails without damage to the wood, machine, or you.

If you are only resurfacing the beams, then invest/rent a surface sander rather than using a planer. As surface sander is just as it sounds, it uses abrasive belts to do the work instead of spinning blades, much safer for applications like this than traditional surfacing tools. Keep in mind that you're not only dealing with potential metal in the wood, you've got years of dirt, dust, and grit embedded in the material, possibly a rock or too, none of which will hurt a sander, but will destroy planer knives and other cutting type tools in short order.